Ergonomics and Safety of Intelligent Driver Interfaces

tion for a moving target is a more complicated task than the same judgment
for a stationary target. Using the Gibsonian model for TTC judgment (e.g., Lee, 1976), in the case of a moving target, the optic flow originating from
the target is inconsistent with that originating from the background scene.
The estimation of relative motion and the subsequent calculation of required braking point based on this information will therefore be more
difficult. It was noticeable that the variability in subjects' brake points was
significantly higher for slow relative speeds (i.e., faster targets) than for
fast speeds of approach (slower or stationary targets). This inconsistency
can be taken to support the previous hypothesis. If moving target situations
are more complex, the results could be due to the calculation of relative
motion simply taking longer, leading to later braking decisions.

CONCLUSIONS

There are several implications of the findings of the first study to the
design of collision avoidance headway displays. First, given that only the
abstract visual warning achieved better performance results than the nondisplay-mediated control, and that in general it was liked by subjects, it is
recommended that this type of visual warning display should be employed.
The display should also provide continuously available feedback as the one
used in this study. It should be noted that the visual displays used in the
experiment were presented in head-up mode. The assumption was that if
one display was more effective than the others, then it was certain to be
apparent in this superior presentation mode. Of course, there is no guarantee that the previous findings can be extrapolated to other modes of
presentation (i.e., midhead and head-down displays).

Second, in collision avoidance systems dedicated to a particular type of
hazard (e.g., a headway warning system), the use of a discrete nonspeech
auditory warning combined with the visual display is recommended. This
conclusion is based on the grounds that subjects clearly preferred the
nonspeech warnings. Also, whereas this type of display was not shown to
result in braking responses that were significantly earlier than the nondisplay-mediated control, it incurred fewer collisions. Third, where an integrated collision avoidance system is to be designed the use of speech
warnings is advisable. Bertone ( 1982) noted that speech warnings are more
informative than simple auditory tones, and they not only alert the user
to the problem but also provide more cues as to its nature. The fact that
language is highly overlearned means that speech is likely to be more
effective in conditions of high workload or stress, where the meaning of
coded auditory tones may be forgotten ( Edman, 1982). As stated previously,
a number of experimental factors contributed to the unpopularity of the

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